(1) Field of the Invention
The present invention relates to a method for removing the iron content contained in a petroleum series mineral oil fraction.
(2) Description of the Prior Art
In the industrial field of petroleum refining, generally a crude oil is fractionated to separate it into a gasoline fraction, a kerosene fraction, a gas oil fraction and an atmospheric distillation residual oil fraction. These petroleum hydrocarbon fractions can be each refined to make petroleum products such as gasoline, kerosene, gas oil and fuel oil. A refining process which is now employed most usually is the so-called hydrogenation refining process in which hydrogen is allowed to react with the petroleum hydrocarbon fractions in the presence of a catalyst under a high pressure at a high temperature.
Impurities present in the petroleum fraction which is a raw material for the hydrogenation refining process contain or an extremely low concentration (e.g., 1 pmm or less) of iron content, when the petroleum fraction is a distillate fraction. However, the raw material distillates containing an iron content as much as 1 pmm or more are prepared on occasion, and some of such material oils contain 100 pmm or more of the iron content.
In many cases, such a material oil has a high acid value, and thus at least a part of the iron content contained therein can be considered to be that which is dissolved in the oil as a result of corrosion in a distilling plant, a storage tank for the distillate, oil transport pipes and the like by acidic materials present therein.
When such a material containing the higher iron content is treated by the hydrogenation refining process, a troublesome problem will occur in the operation of a refining plant. That is, since they are dissolved in the material oil, compounds containing the iron content are not caught by a filter disposed at an inlet of the plant, get into the plant, and reach the reactor which is an important portion of the plant, where the compounds are decomposed due to chemical reactions. The iron content is deposited in the form of sulfides between catalytic grains in the reactor and clog the reactor or to adhere to the surfaces of the catalytic grains, with the result that the power of a catalyst will be lowered. In the case that such a material oil is treated, the distilling plant, the distillate storage tank, the pipes and the like are manufactured with anti-corrosive materials, or alternatively the insides of these members may be subjected to a lining treatment with anti-corrosive materials, as known. In this measure, however, it is necessary to give the anticorrosion treatment to the huge storage tank and the long oil transport pipes, which increases costs.
As another process, a guard reactor is disposed upstream of the reactor in the hydrogenation refining plant.
In this arrangement, the material oil is introduced into a reactor which is called the guard reactor, where the iron content dissolved in the oil is decomposed by chemical reactions. The iron content is then changed into sulfides, and the latter are caught by a filler and catalytic grains with which the guard reactor is filled, and are deposited in the guard reactor.
The material oil from which the iron content has been removed in this way is caused to leave the guard reactor for the reactor.
According to this process, the clogging of the reactor and the deterioration in the used catalyst can be prevented, but the clogging of the guard reactor and the decline of the catalyst, by the iron compounds, with which the guard reactor is filled are unavoidable.
Still another process is composed of treating the distillate containing the iron content with an aqueous sodium hydroxide solution to neutralize the acidic materials present in the oil and extracting them into an aqueous solution.
In this process, the iron content is in the form of hydroxides or oxides, and the latter are transferred into the aqueous solution or are collected on each interface between the aqueous solution phase and the oil phase. This process is now utilized as a manner for extracting naphthenic acid from the petroleum fraction containing the acid. If the recovered naphthenic acid has any commercial value, the above mentioned process is advantageous.
However, this process is not practicable for a high-viscosity oil because of the difficulty of the separation between the oil phase and the aqueous solution phase and poor extraction of a naphthenate into the aqueous solution, although it is not impossible when a viscosity of the material oil fraction is low, for example, as in the case of a spindle oil fraction.
As still another process, the petroleum series mineral oil is brought into contact with hydrogen sulfide or ammonia in order to convert the iron content in the mineral oil into iron compounds which are insoluble in the oil, and the thus formed iron compounds are then removed by filtration, centrifugation or the like. However, particles deposited in the petroleum series mineral oil principally have a particle diameter of 10 microns or less, or a much small particle diameter than 1 micron on occasion. In consequence, the usual fine mesh filter cannot catch and remove such particles. One can consider making use of a membrane or a filter paper, but in the treatment of a great deal of the petroleum series mineral oil, such a material will be clogged with the particles to increase pressure and to break it, and additionally troublesome exchange is necessary. In short, such a material as the membrane or the filter cannot be employed on an industrial scale, though its employment is possible on a laboratory scale. Further, a method of using the technique of centrifugalization is also present, but it is not practicable either from the viewpoints of structure and operation.
On the other hand, the atmospheric distillation or the vacuum distillation residual oil contains, as the iron content, a large number of fine particles comprising iron and/or iron compounds. These iron particles are those which have come from a tank, pipes and a plant owing to erosion, when the crude oil is transported from a producing center by a tanker, is then stored in the tank, and is afterward delivered to the distilling plant via the oil transport pipes.
If such a distillation residual oil is used as the material oil for a fixed-bed type hydrogenation plant, the fine iron particles contained in the material oil will be deposited on the catalyst or between catalytic grains in a reactor, so that the latter will be clogged therewith or an activity of the catalytic grains will declined. Particularly, in the case of the clogging of the reactor, the pressure in the plant will be thereby increased, and at times, the operation of the plant must be stopped, which fact will give rise to an extremely large economical loss.
It is thus apparent that if the fine particles comprising the iron content are removed from the material oil, a great advantage will be obtained. However, such fine particles are as extremely small as about 0.1 to about 20 microns and thus cannot be removed by a filter which is generally employed in the petroleum refining industry.
In addition to the above mentioned techniques, several other processes for removing the fine particles can be enumerated. For example, a filter paper or a membrane having a fine mesh can be used as a filter. In the case that such a filter is employed, however, the pressure loss is very large, and clogging is liable to occur, which facts scarcely permit the filter to be used for a long period of time. In addition, since it is necessary that the old filter is exchanged for a new one, the manner of using the filter is indeed unsuitable for the treatment of a large amount of the material oil from the viewpoint of the operation.
Another method is characterized by the employment of centrifugalization, but it is poor in throughput in view of structure and operation and consequently is less practicable.